iHerp Australia Issue 7 | Page 54

Conservation biologist and author Kit Prendergast examines a physiological feat unique amongst reptiles .

R eptiles are often described as ‘ cold-blooded ’, however this scientifically inaccurate term is more appropriately replaced with ectothermic ’, which better describes their thermoregulatory strategy . They rely on behavioural means to regulate their body temperature , which is thus largely determined by the temperature of their environment . This contrasts with ‘ endothermic ’ animals , which have evolved a thermoregulatory strategy whereby their bodies are maintained at a relatively high , constant temperature independent of the environment through endogenous heat production ; that is , through metabolic activity , which generates heat as a by-product . Usually , we classify herps ( non-avian reptiles and amphibians ) as ectothermic , and mammals and birds as endothermic . But , as with most things in nature , there are often exceptions to the classifications and categories we erect …. in this case , endothermic snakes ! when we ’ re cold , we have the ability to automatically increase heat production through shivering , which is muscular activity performed purely to generate heat . However , mammals and birds are not the only animals capable of generating heat through shivering thermogenesis – pythons can do it too .

Most of the time , pythons are your typical reptiles : their body temperature largely conforms to that of their environment . When it ’ s cold they ’ re cold , and will seek sunny areas in which to bask and rocks heated by the sun , to warm up through radiative and conductive heat exchange respectively . Most reptiles , being ectothermic , cannot incubate their eggs , which are typically laid in nests dug in areas that receive warmth from the sun . But a special exception occurs for brooding female pythons , which can incubate their eggs by heat generated through shivering .
For endotherms like ourselves , as well as other birds and mammals , one way to ramp up our heat production when we ’ re cold is by shivering ; rapidly contracting the body muscles in a physiological activity described as ‘ shivering thermogenesis ’. Muscular activity generates significant heat , and usually when we engage in physical activity , this heat production is a ‘ waste ’ of energy : we want all the energy generated through our metabolic activity to do ‘ useful ’ work like run , jump , swim or cycle . But
Shivering has been well-studied in one of the best known of Aussie pythons , the Carpet Python ( Morelia spilota ), and particularly the Diamond Python ( M . s . spilota ). Peter Harlow and Gordon Grigg were the first to publish their detailed observations and measurements of a brooding female Diamond Python in 1984 . The fact that this phenomenon was only documented fairly recently in one of Australia ’ s most popular reptiles can be put down to a lack of successful captive breeding ( until recently ) coupled with the animals ’ naturally shy disposition . Harlow and Grigg noted that at the slightest disturbance – visual or vibrational – their female would cease shivering .
Whilst brooding , their captive python raised her body temperature to 32-33 o C , which was as much as almost 7 o C higher than ambient temperatures . She achieved this by repeated shivers ( up to 50 per minute ), each consisting of a series of muscular contractions involving the whole body . There was a definite daily rhythm to this process , with shivering rate , oxygen consumption and thus body temperature all peaking in the late